1. Field Of The Invention
The invention relates to solvent dewaxing a petroleum derived lubricating oil stock. More particularly, the invention relates to the production of dewaxing solvent free lubricating oils by steam stripping. Most particularly, the invention relates to producing lubricating oils which are free of water haze on standing at subambient temperature.
2. Description Of the Related Arts
Waxy petroleum oil stocks are solvent dewaxed by cooling oil-solvent admixtures under controlled conditions for crystallization of wax from the admixtures. Commercially, such oil-solvent admixtures are cooled by several methods such as indirect heat exchange in scraped surface exchangers; dilution chilling wherein waxy oil stock is contacted in a multistage tower with chilled solvent under conditions of high levels of agitation, and direct chilling, wherein a low boiling solvent, e.g. propylene, mixed with waxy oil stock is vaporized under conditions of reduced pressure.
In such commercial processes, the waxy oil charge, or mixtures of waxy oil charge and dewaxing solvent, are heated to a temperature at which the wax is dissolved. The heated charge is then passed into a cooling zone wherein cooling is undertaken at a uniform slow rate in the range of about 1.degree. to 8.degree. F./min. (0.56.degree. to 4.4.degree. C./min.) until a temperature is reached at which a substantial portion of the wax is crystallized and at which dewaxed oil product has a selected pour point temperature. Upon achieving the desired dewaxing temperature, the mixture of wax crystals, oil and solvent is subjected to solid-liquid separation for recovery of a wax free oil-solvent solution and a solid wax containing a minor proportion of oil (slack-wax). The separated oil-solvent solution is subjected to distillation for recovery of a solvent fraction and a dewaxed oil product fraction. The slack wax may be recovered as is, or may be subjected to additional processing, such as repulp filtration for removal of additional oil therefrom.
Solid-liquid separation techniques which may be employed for separation of wax crystals from the oil-solvent solutions include known solid-liquid separation processes, such as gravity settling, centrifugation, and filtration. Most commonly, in commercial processes, filtration in a rotary vacuum filter, followed by solvent wash of the wax cake, is employed.
Dewaxing solvents which may be used in solvent dewaxing processes include known dewaxing solvents. Commonly used solvents include aliphatic ketones of 3-6 carbon atoms, C.sub.2 -C.sub.4 range hydrocarbons, C.sub.6 -C.sub.7 aromatic hydrocarbons, halogenated C.sub.1 -C.sub.4 hydrocarbons, and mixtures of such solvents.
Solvents known to be useful as dewaxing solvents are the ketones containing 3 to 6 carbon atoms, for example, acetone, methylethylketone (MEK) and methylisobutylketone (MIBK); mixtures of ketones; and mixtures of ketones with aromatic hydrocarbons including benzene and toluene. Halogenated low molecular weight hydrocarbons, including dichloromethane and dichloroethane, and their mixtures are also known dewaxing solvents. Solvent dilution of waxy oil stocks maintains fluidity of the oil for facilitating easy handling, for obtaining optimum wax-oil separation, and for obtaining optimum dewaxed oil yields. The extent of solvent dilution depends upon the particular oil stocks and solvents used, the approach to filtration temperature in the cooling zone and the desired final ratio of solvent to oil in the separation zone.
For processes employing indirect cooling in scraped surface exchangers, cooling and wax crystallization are accomplished under conditions of very little agitation at a cooling rate in the range of about 1.degree. to 8.degree. F./min. (0.56.degree. to 4.4.degree. C./min. Under such conditions, without wall scrapers, wax tends to accumulate on the cold exchanger walls, interfering with heat transfer and causing increased pressure drop. Thus, wall scrapers are employed to remove the accumulated wax. Dewaxing solvents are employed to maintain fluidity of the oil in the coolers, and may be added before the oil is cooled or in increments during cooling. Often the oil is given a final dilution with solvent at the separation temperature for reducing solution viscosity such that wax separation is more efficient. Commonly, solvent added to the oil in such processes is at the same temperature, or somewhat higher temperature than the oil. Cold solvent, added at substantially lower temperatures than the oil, shock chills the oil resulting in formation of many small wax crystals which can be difficult to separate. Under controlled conditions, elongated wax crystals of good size are formed which are easy to separate and which contain little occlude oil.
Dilution chilling processes employ incremental addition of cold solvent, e.g. to +20.degree. to -25.degree. (-6.7.degree. to -32.degree. C.) to the oil at high degrees of agitation such that oil and solvent are completely mixed in less than one second. Under such conditions, wax precipitates in small, hard balls rather than elongated crystals. Such wax precipitates are easy to separate and retain very little oil.
Direct chilling processes employ a low boiling hydrocarbon, e.g. propylene, as dewaxing solvent and refrigerant. Waxy oil stock is diluted with sufficient low boiling hydrocarbon to provide the necessary cooling and provide the desired final dilution for separation of solid-wax from the oil-solvent solution. The low boiling hydrocarbon is vaporized from the oil-low boiling hydrocarbon solution, under conditions of reduced pressure, at a rate sufficient to cool the solution about 1.degree. to 8.degree. F./min. (0.56.degree. to 4.4.degree. C./min). Such cooling is continued until the desired separation temperature and amount of wax crystallization are obtained. At the separation temperature, sufficient low boiling hydrocarbon remains in solution with the oil to provide the desired fluidity for separation of wax. Agitation of the mixture being cooled is commonly provided for reduction of temperature and concentration gradients.
U.S. Pat. No. 3,972,779 to C. W. Harrison teaches a system for controlling a dewaxing apparatus. The system comprises a viscosity measurement of the waxy oil-wax crystal slurry mix and a refractive index measurement of the solvent to calculate incremental changes to dilution of wax oil with solvent.
U.S. Pat. No. 4,199,433 to H. 0. Button et al. teaches a continuous process for solvent dewaxing lubricating oil stocks. The objective of the process is to eliminate the steam stripping of products. In the process, an inert gas, such as nitrogen, is used as a stripping agent for removing solvent from dewaxed oils, deoiled waxes and slack waxes resulting from solvent dewaxing processes.
Petroleum Refiner, Vol. 15, No. 6, June 1936, pp. 205-209 discusses the useful art of solvent dewaxing and the terminology used by those trained in the art.